1. Field of the Invention
The present invention relates to films having poly(vinyl alcohol) coatings, and, more particularly, to enhancing the oxygen barrier properties and processing characteristics of cross-linked poly(vinyl alcohol) coatings.
2. Background of the Related Art
Poly(vinyl alcohol) coatings have been applied to various substrates in the past, and are known to provide a barrier to the transmission of oxygen. However, poly(vinyl alcohol) is soluble in water and as a result susceptible to attack by moisture. Various attempts have been made in the prior art to decrease the sensitivity of poly(vinyl alcohol) to moisture.
As known to those skilled in the art, poly(vinyl alcohol) (PVOH) is typically produced by hydrolyzing poly(vinyl acetate). Specifically, the acetate groups of poly(vinyl acetate) are replaced with alcohol groups through a hydrolysis reaction. The more acetate groups that are replaced, the greater the hydrolysis of the PVOH resin. For example, in a 95% hydrolyzed PVOH resin approximately 5% of the acetate groups remain unchanged. Similarly, in a 99% hydrolyzed PVOH resin, approximately 1% of the acetate groups remain unchanged.
One known method of decreasing the sensitivity of a PVOH coating to moisture is to cross-link the PVOH. For example, a cross-linking agent and catalyst may be applied along with the PVOH such that the agent interconnects and thereby cross-links the PVOH molecules as the coating dries. The catalyst is present to aid in the cross-linking process which otherwise may not occur without it.
Commonly owned co-pending Application Ser. No. 08/221,558, now abandoned discloses a polymeric substrate having a cross-linked layer of PVOH adhered thereto. The PVOH layer includes a cross-linking agent, such as melamine-formaldehyde or urea-formaldehyde, which cross-links the PVOH in the presence of an acid catalyst. The resultant film exhibits enhanced oxygen barrier characteristics, as compared to barrier films of the prior art.
Another method of decreasing the sensitivity and thereby fortifying the oxygen barrier characteristics of a PVOH coating is by utilizing a highly hydrolyzed PVOH resin. It is believed that the presence of more alcohol groups, i.e., greater hydrolysis, directly corresponds to better barrier properties and better moisture resistance. As a result, many of the PVOH coatings found in the prior art utilize PVOH resins having a degree of hydrolysis 98% or greater.
However, highly hydrolyzed PVOH resins have been found to create difficulties during application as a coating. Highly hydrolyzed PVOH resins exhibit a greater tendency to foam when pumped. Consequently, microscopic bubbles are created in the coating. This results in imperfections in the PVOH coating which may reduce the coating's moisture resistance and impermeability to oxygen.
Further, highly hydrolyzed PVOH resins exhibit a poorer "wet out" characteristic on a substrate as compared to the "wet out" exhibited using a less hydrolyzed PVOH resin. "Wet out" means the tendency of a coating to spread over a substrate. Highly hydrolyzed PVOH is less efficient during spreading than is less hydrolyzed PVOH.
Therefore, there is a need in the art for PVOH coatings which exhibit a high degree of oxygen impermeability and moisture resistance, while at the same time are easier to apply. Specifically, there is a need for PVOH coatings having the moisture resistance and high impermeability associated with highly hydrolyzed PVOH resins while, at the same time, possessing the lower foaming tendency and the better wet out associated with less hydrolyzed PVOH resins.
It is an object of the present invention to provide PVOH coatings which exhibit oxygen impermeability and moisture resistance associated with highly hydrolyzed PVOH resins, while at the same time, exhibiting lower foaming tendency and the better wet out associated with less hydrolyzed PVOH resins. It is also an object of the present invention to provide a low oxygen transmissive film utilizing this coating.